1. Field of the Invention
The present invention relates to hand lever apparatus for operating a driven member such as, for example, a throttle valve of an internal combustion engine through a cable and more particularly, to a hand lever apparatus arranged near a grip portion of a handle of a work machine such as a grass trimmer or a hedge trimmer and suited for operating the throttle valve through a throttle cable or the like.
2. Description of the Prior Art
Conventionally, in a work machine such as a grass trimmer or a hedge trimmer in which a working unit including a cutter is generally driven by an internal combustion engine, and which has the aim of adjusting the output of the driving internal combustion engine by hand, a hand lever apparatus for adjusting of the opening of a throttle valve of the internal combustion engine is arranged near a grip portion of a U-shaped handle or a bar handle provided for the work machine.
The hand lever apparatus has a throttle trigger (throttle lever) typically operated by fingers of an operator. By rotationally operating the throttle lever, the opening of the throttle valve can be adjusted through the throttle cable. The throttle valve is normally biased to a minimum opening (idle rotation opening) so that the idle rotation opening may be maintained under the intact condition in which the throttle lever is not operated when the throttle lever is operated to pull the throttle cable by a predetermined length or more to cancel out a play, the opening may be increased from the idle rotation opening to an opening for a high-speed rotation.
Two types of throttle lever apparatuses for adjusting of the opening of the throttle valve are generally known. One is an automatic idle rotation opening restoring type in which when the throttle lever is released from the rotationally operated state, the lever together with the throttle valve is automatically returned to the intact position (idle rotation opening. The other type is an idle opening non-restoring type in which when the fingers operator's are removed merely from the throttle lever, the throttle lever is not moved but can be constantly maintained at a desired rotationally operated position (see, e.g., Japanese Utility Model Publication No. 57-19944).
In the automatic idle rotation opening restoring type, when the operator's fingers are removed from the throttle lever during operation of the work machine driven by the internal combustion engine, the internal combustion engine automatically returns to the idling state. In the case where the work machine transmits power from the internal combustion engine to a working unit including a cutter through a centrifugal clutch, the centrifugal clutch is cut off to interrupt power transmission to the working unit under the idling state in which the rotary driving force from the internal combustion engine is decreased. Accordingly, in the event that an accident occurs during the operation of the work machine and power transmission to the working unit including the cutter must be interrupted, the throttle valve can be returned immediately to the idle rotation opening by only releasing fingers from the throttle lever. Thus, the driving of the working unit can be stopped, thereby providing that safety during operating the working unit.
In the automatic idle rotation opening restoring type, an automatic return to the idling state can be ensured by only removing the operator's fingers from the throttle lever. However a desired rotationally operated position of the throttle lever must be maintained by constantly holding or gripping the throttle lever by the opeator's fingers.
The throttle cable connecting the throttle valve and the throttle lever is tortuous in many directions on the way to the throttle lever from the throttle valve. Moreover, a return spring of the throttle valve is required to have enough strength to return the throttle valve. Accordingly, in order to maintain a desired rotationally operated position by holding the throttle lever by the operator's fingers during operation of the work machine, a force for opposing the throttle cable and tension of the return spring is needed. Unfortunately, when the throttle lever is gripped by the operator's fingers for a long time, the operator's fingers become fatigued.
More particularly, the automatic idle rotation opening restoring type requires fingers to exert strong (highly depressive) holding force on the throttle lever during its normal operation. Thus difficulties with ease of operation arise in spite of the aforementioned advantage of high safety.
In addition, it is difficult for the automatic idle rotation opening restoring type to hold the throttle valve in an intermediate opening position. More particularly, the throttle valve can be held in an intermediate opening position by keeping the gripping force exerted on the throttle lever by the operator's fingers evenly. Therefore even when the holding force is slightly changed, the opening of the throttle valve is changed by a corresponding amount and as a result, the rotational speed of the internal combustion engine is changed. Accordingly, as soon as the operator's fingers gripping the throttle lever becomes fatigued during operation the throttle lever gripped by the operator's fingers and the gripping force is weakened, the rotational speed of the internal combustion engine is changed and a desired output (rotational speed) suitable for the working unit can hardly be obtained.